Sintered gears are generally manufactured as follows: a metal powder is filled in a mold, which is compressed by the use of a punch in the vertical direction to form a compact; and the compact obtained in the shape of a gear is sintered. Since such a sintered gear can be manufactured in large quantities at low costs, sintered gears of various applications are commercially available as varied products. In the sintered gear, pores remain in a metal matrix due to the manufacturing method. Accordingly, density of the material forming the gear is lower than real density. This leads to a problem that strength of the gear is reduced in comparison with a general gear made of wrought steel having real density. Conventionally, this problem is addressed by: use of a higher grade material than the material of the wrought steel gear, to make up for a deficiency in strength; employing a 2P-2S method in which pressing and sintering are performed twice (pressing, provisional sintering, secondary pressing, and sintering) in the manufacturing process; or applying warm pressing technique of pressing the powder in a warm environment or the like for densification.
In recent years, however, the need for lower cost has been increasing. Use of a high-grade material and application of complicated manufacturing process are extremely disadvantageous in meeting the need and it makes difficult to solve the problem. For this reason, a sintered gear having a high strength that can be manufactured by using inexpensive material and simple manufacturing method has been much sought after.
In this connection, it has been proposed that, in the gear formed by pressing and sintering a powdered metal material, the teeth, tooth root and flank regions of the gear are surface-hardened to establish densification ranging between 90 to 100% up to the depth of at least 380 microns (Refer to International Patent Application Japanese National Publication No. H6-501988, pages 2 to 3, FIG. 1, for example).
On the other hand, the gear is a member for transmitting power by engaging with the other member's teeth or a chain and rotating together, and the contact and friction on the engaging portion of the gear makes noise. The need to suppress this noise and improve noise reduction characteristics in gears has also been increasing.
An object of the present invention is to meet the above-mentioned needs and provide a sintered gear that can be easily manufactured at lower cost and has good noise reduction characteristics.